Microtidal Sandy Beaches Research Articles

Multicausal environmental severity: A flexible framework for microtidal sandy beaches and the role of polychaetes as an indicator taxon

The aim of this paper is to study the macrofaunal community dynamics and the biological–environmental interactions in the mid- and sublittoral ecosystems of the microtidal Mediterranean sandy shores. Four sandy beaches, three on the island of Crete and one on the northwest coast of Italy were selected to investigate the spatial and temporal changes in the community structure and the associated environmental variables. The littoral zone, which has not been adequately studied in the Eastern Mediterranean, presents special interest not only from the scientific point of view but also for practical reasons of ecological management. The multivariate techniques revealed that the community pattern of the sandy beach macrofauna is mainly spatial rather than temporal. There are pronounced differences in species composition and abundance of the macrofaunal assemblages of the mid- and sublittoral zone. The multicausal environmental severity hypothesis appears to be valid for the sandy beach macrofaunal communities of the Mediterranean. The abundance and composition of the macrofaunal assemblages are highly variable and are affected by the synergistic effects of many environmental variables. The polychaete taxonomic assemblage structure closely follows the macrofaunal community pattern. Differences between the two patterns may arise from the different responses that polychaetes may show to the environmental stress.

Composition, structure and zonation of intertidal macrofauna in relation to physical factors in microtidal sandy beaches at Río de Janeiro State, Brazil

SUMMARY: The intertidal macroinfauna of fiftheen exposed sandy beaches located in Rio de Janeiro State was studied to determine species richness, density, biomass and zonation. Morphodynamic influence on biological parameters is also evaluated in this paper. Dean’s morphodynamic index (Ω) ranged from reflective to intermediate values and mean grain size ranged from fine to coarse sands. A total of 17 species were identified, with the crustaceans Emerita brasiliensis, Excirolana braziliensis and Pseudorchestoidea brasiliensis being the most abundant species. Species richness had a significant positive linear relationship with Ω values, swash period and slope, and a negative relationship with grain size. Significant positive relationships were also observed for total density with the swash period and slope, total biomass with slope, density of Emerita brasiliensis with swash period and slope and biomass of Emerita brasiliensis with slope. Zonation analysis showed that two or three biological zones could be recognised, with the two-zone pattern being observed in most of the beaches. This pattern consisted of a lower zone characterised by Emerita brasiliensis, Donax haleyanus and Hemipodus oliveri; and an upper zone characterised by Excirolana braziliensis, Pseudorchestoidea brasiliensis and Phaleria testacea. The threezone pattern as a disruption of the general two-zone pattern and its formation was attributable to the variation in the distributions of P. brasilensis and E. braziliensis. Our results suggest that beach morphodynamics is the best predictor for the variability of macroinfaunal community composition.

The role of bore collapse and local shear stresses on the spatial distribution of sediment load in the uprush of an intermediate-state beach

Spatial variations in sediment load in the swash uprush and textural properties of sediment in transport were evaluated to investigate the mechanisms responsible for sediment transport during wave uprush. Four streamer traps were deployed at 2.0-m intervals across the swash zone of a sheltered, microtidal sandy beach at Port Beach, Western Australia, over a 4-day period. During these trapping experiments, offshore significant wave heights were 0.3–0.5 m and wave periods were about 10 s. The average width of the uprush zone was 6.9 m and the average uprush duration was 5.9 s. Cross-shore distributions of sediment load for 70 uprush events reveal a maximum in sediment load landward of the base of the swash (at about 20% of swash width) during single events and a maximum closer to mid-swash (at about 40% of swash width) during multiple events characterized by swash interactions. Settling velocity distributions of trap samples during individual uprush events are similar to distributions found on the beach surface, with the lowest settling velocities (finest sediments) near the base of the swash zone and maximum settling velocities (coarsest sediments) around the mid-swash position. It was found that sediment transport during wave uprush occurs through two distinct mechanisms: (1) sediment entrainment during bore collapse seaward of the base of the swash zone and subsequent advection of this bore-entrained sediment up the beach by wave uprush; and (2) in situ sediment entrainment and transport induced by local shear stresses during wave uprush. Both mechanisms are considered important, but the first mechanism is considered most significant during the early stages of wave uprush when sediment is transported mainly in suspension, while the second mechanism is likely to dominate the mid- to later stages of wave uprush when sediment is transported mainly by sheet flow. The relative importance of the two mechanisms will vary between different beaches with the morphodynamic state of the beach (reflective versus dissipative) expected to play a major role.

Activity rhythms of two cirolanid isopods from an exposed microtidal sandy beach in Uruguay

Activity rhythms of two cirolanid isopods, Excirolana armata and Excirolana braziliensis, were studied based on both seasonal field observations and laboratory experiments, at an exposed microtidal sandy beach in Uruguay. The natural emergence patterns of both species were observed in the field for 1 year, twice in each season, and correlated to sea level, expected tidal cycles and diel cycles. Laboratory experiments were carried out in order to detect endogenous rhythms of activity and observe how emergence of both species was affected by changes in light and/or sediment thixotropy. We also compared behavioral strategies of sympatric species that occupy different beach levels. Sea level (and thus swash zone position) during field sampling did not follow expected tidal cycles for most sampling occasions. E. armata was observed in activity most of the time, but activity only correlated with sea level on three out of eight occasions, and only once was correlated to expected tidal cycle. Laboratory results showed that emergence under constant conditions was rare; changes in sediment thixotropy stimulated emergence, but the response was not cyclical; light had little effect on this response. On the other hand, E. braziliensis was fairly scarce in the water column, but swimming individuals were observed always during the night. They displayed an endogenous circadian activity pattern in the laboratory which augmented in response to changes in sediment thixotropy. The natural light/dark cycle modulated both spontaneous and response emergence by increasing day/night differences in activity. In this study E. armata, a midlittoral species more exposed to sea level variations, seemed to rely entirely on different physical and/or biological cues to trigger emergence at the appropriate time. E. braziliensis, found mostly in the upper intertidal zone, emerged in a circadian rhythm, which was stimulated by changes in sediment thixotropy and reinforced by light cycles. The results of this study led us to conclude that on microtidal, unpredictable beaches, local physical and biological factors can combine to determine different activity strategies in organisms from different intertidal levels.

Lessons from a Disturbance Experiment in the Intertidal Zone of an Exposed Sandy Beach

Exposed sandy beaches are important, sensitive and widespread coastal habitats. Although they have been studied for more than 50 years, investigators have been reluctant to attempt manipulative experiments due to the dynamic nature of these environments. Consequently, the ecology of exposed sandy beaches remains relatively poorly understood. We conducted a community-level, manipulative experiment involving a simulated anthropogenic disturbance on an exposed microtidal sandy beach in the Eastern Cape, South Africa; the first of its kind and scale. This study comprised pre- and post-impact sampling at an experimental site and two control sites. The impact involved excavating and removing a 200m2quadrat of sand from the mid-intertidal of the experimental site to a depth of 0·3m. The intention was to address the prediction that anthropogenic disturbances would be detectable if appropriate spatial and temporal scales were investigated. The following variables were monitored: transect gradient; species richness; macrofaunal abundance; and both the abundance and biomass of the dominant infaunal species, the beach clam Donax serra Röding. Analyses revealed significant differences in temporal patterns of all response variables amongst sites. Some evidence linked these changes to the experimental disturbance, although impacts appear temporary, being ameliorated within, at most, one semi-lunar cycle. This confirms that it is possible to successfully conduct manipulative experiments on exposed sandy beaches. However, the uncontrollable, natural dynamics of the beach face, as expressed by intertidal gradient, contributed significantly to the description of spatio-temporal variation in biotic response variables. It is concluded that to isolate treatment effects from those of natural variation, two advances are necessary on the current research approach. First, experimental designs must take cognizance of the fact that exposed, microtidal sandy beaches have little in common with other intertidal habitats; and second, large-scale treatments must be replicated in space.

Validation of a Boussinesq model of beach ground water behaviour

A numerical solution to the 1-D Boussinesq ground water equation is used to predict water table response to tidal and wave forcing. We argue that the solution presented here offers considerable advantages over other solutions to the 1-D Boussinesq ground water equation and is simpler to apply than 2-D ground water models. The main assumption in using the Boussinesq equation is that ground water flow in a shallow aquifer can be described using the Dupuit-Forchheimer approximation, in which it is assumed that changes in hydraulic head with depth below the water table are negligible. The predictive ability of the solution was tested against field data from ten wells and ten piezometers on a microtidal sandy beach. Two model runs were carried out: in the first the effect of tides only was considered; in the second setup due to waves was also considered. The arithmetic mean of independently measured values of hydraulic conductivity was used to describe sediment characteristics. The piezometer data show that changes in hydraulic head with depth are negligible for much of the beach, suggesting that the use of the Boussinesq equation is appropriate. The correspondence between model predictions of water table elevations and field observations is good for both the tide-only and the tide plus setup model runs, with the latter giving the better predictions.

Macroinfauna Zonation in Microtidal Sandy Beaches: is it Possible to Identify Patterns in Such Variable Environments?

Prior studies on sandy beach macroinfauna have been directed to elucidate generalizable zonation patterns. These studies, usually based on a reduced temporal scale (i.e. one sampling date), have often highlighted the discrepancies between their results and the classical, static zonation schemes. This paper demonstrates that snapshot studies might fail to obtain conclusive results about zonation patterns in microtidal sandy beaches, because these constitute dynamic environments subjected to many sources of uncertainty. For this purpose, a yearly study (February 1988–January 1989) of the macroinfauna was carried out on a dissipative beach on the Atlantic coast of Uruguay. An average zonation pattern with three main belts was recognized between the sand dunes and the lower levels of the swash zone when considering annual mean abundances. This pattern roughly matched traditional zonation schemes, whereas monthly patterns did not always fit them—with the exception of some species. Important spatial variability of the macroinfauna was observed, with aperiodic and seasonal components. The former was associated with unpredictable movements of tidal levels, with the macroinfauna changing its position, tracking the high water level. Concerning seasonal periodicity, the species tended to occupy the upper levels of their respective distribution areas during spring and summer, whereas in autumn and winter they inhabited the lower levels. The fact that faunal zones are dynamic, and track sea levels and seasons, diminishes the applicability of static traditional patterns. It was concluded that a yearly study is needed to give a more representative picture of the zonation patterns in microtidal sandy beaches.

The relationship of juvenile southern mole crabs Hippa australis Hale (Crustacea : Anamura : Hippidae) to surficial swash water-circulation over several diurnal spring-tide cycles during winter conditions on a micro-tidal sandy beach

The location of the southern mole crab Hippa australis Hale within the swash zone at North Beach, Western Australia, was studied with respect to tidal levels, wave run-up, beach morphology, and wave energy conditions over a 4-day period. H. australis undertake distinct tidal migrations within the ground-water resurgence zone on the sandy beachface where characteristics of the water circulation system alter with tidal fluctuations. They locate themselves high in the swash zone at low tide and low in the swash zone at high tide. Mole crabs avoid high wave energy conditions as indicated by their scarcity during storms at North Beach. Their immediate reappearance in higher number as storm surges subside further indicates that they avoid storms by burrowing deep into the substratum. The position of mole crabs at any time during the tidal cycle may be related to the opportunities of feeding in the swash zone.